Meninges
The meninges is the system of membranes which envelops the central nervous system. In mammals, the meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater. The primary function of the meninges and of the cerebrospinal fluid is to protect the central nervous system. Meninges ( OED 2nd edition, 1989.Entry "meninges" in Merriam-Webster Online Dictionary, retrieved 2012-07-28.) is the plural of meninx ( or Entry "meninx" in Merriam-Webster Online Dictionary, retrieved 2012-07-28.), from .μήνιγξ, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus. The adjective form is meningeal ( ). Anatomy Dura mater The dura mater 'tough mother' (also rarely called meninx fibrosa or pachymeninx) is a thick, durable membrane, closest to the skull. It consists of two layers, the periosteal layer which lies closest to the calvaria (skull), and the inner meningeal layer which lies closer to the brain. It contains larger blood vessels which split into the capillaries in the pia mater. It is composed of dense fibrous tissue, and its inner surface is covered by flattened cells like those present on the surfaces of the pia mater and arachnoid. The dura mater is a sac which envelops the arachnoid and has been modified to serve several functions. The dura mater surrounds and supports the large venous channels (dural sinuses) carrying blood from the brain toward the heart. The dura has four areas of infolding which include : * Falx cerebri, the largest, sickle-shaped; separates the cerebral hemispheres. Starts from the frontal crest of frontal bone and the crista galli running to the internal occipital protuberance. * Tentorium cerebelli, the second largest, crescent-shaped; separates the occipital lobes from cerebellum. The falx cerebri attaches to it giving a tentlike appearance. * Falx cerebelli, vertical infolding; lies inferior to the tentorium cerebelli, separating the cerebellar hemispheres. * Diaphragma sellae, smallest infolding; covers the pituitary gland and sella turcica. Arachnoid mater The middle element of the meninges is the arachnoid mater, so named because of its spider web-like appearance. It provides a cushioning effect for the central nervous system. The arachnoid mater is a thin, transparent membrane. It is composed of fibrous tissue and, like the pia mater, is covered by flat cells also thought to be impermeable to fluid. The arachnoid does not follow the convolutions of the surface of the brain and so looks like a loosely fitting sac. In the region of the brain, particularly, a large number of fine filaments called arachnoid trabeculae pass from the arachnoid through the subarachnoid space to blend with the tissue of the pia mater. The arachnoid and pia mater are sometimes together called the leptomeninges. Pia mater The pia mater 'soft mother' is a very delicate membrane. It is the meningeal envelope which firmly adheres to the surface of the brain and spinal cord, following the brain's minor contours (gyri and sulci). It is a very thin membrane composed of fibrous tissue covered on its outer surface by a sheet of flat cells thought to be impermeable to fluid. The pia mater is pierced by blood vessels which travel to the brain and spinal cord, and its capillaries are responsible for nourishing the brain. Spaces The subarachnoid space is the space which normally exists between the arachnoid and the pia mater, which is filled with cerebrospinal fluid. Normally, the dura mater is attached to the skull, or to the bones of the vertebral canal in the spinal cord. The arachnoid is attached to the dura mater, while the pia mater is attached to the central nervous system tissue. When the dura mater and the arachnoid separate through injury or illness, the space between them is the subdural space. Pathology There are three types of hemorrhage involving the meninges:Orlando Regional Healthcare, Education and Development. 2004. "Overview of Adult Traumatic Brain Injuries." Retrieved on January 16, 2008. * A subarachnoid hemorrhage is acute bleeding under the arachnoid; it may occur spontaneously or as a result of trauma. * A subdural hematoma is a hematoma (collection of blood) located in a separation of the arachnoid from the dura mater. The small veins which connect the dura mater and the arachnoid are torn, usually during an accident, and blood can leak into this area. * An epidural hematoma similarly may arise after an accident or spontaneously. Other medical conditions which affect the meninges include meningitis (usually from fungal, bacterial, or viral infection) and meningiomas arising from the meninges or from meningeal carcinomatoses (tumors) formed elsewhere in the body which metastasize to the meninges. In other animals In fishes, the meninges is a single membrane (the primitive meninx). In amphibians, reptiles and birds, the meninges include a thick outer dura mater and a thick inner secondary meninx. Mammals retain the dura mater, and the secondary meninx divides into the arachnoid and pia mater.Kardong, Kenneth V. (1995). Vertebrates: Comparative Anatomy, Function, Evolution. Dubuque, Iowa: Wm. C. Brown Publishers. pp. 539. ISBN 0069219917 See also *Membranes Additional images Image:Gray769.png|Diagrammatic representation of a section across the top of the skull Image:Gray1196.png|Diagrammatic section of scalp. Image:Hirnhaut.png References Category:Central nervous system Category:Human head and neck Category:Membranes Category:Meninges